555

Nice. If you’re studying for exams, you might be asking about the 555 timer IC. Here’s a quick refresher you can use.

Key pins

• 1 GND
• 2 TRIG (trigger)
• 3 OUT
• 4 RESET (active low)
• 5 CTRL (control voltage, usually 2/3 Vcc)
• 6 THR (threshold)
• 7 DISCH (discharge transistor)
• 8 Vcc

Operating modes and basic formulas

• Astable (free-running)

  • No trigger needed; outputs a square wave.
  • f ≈ 1.44 / [(R1 + 2R2) C]
  • T_high ≈ 0.693 (R1 + R2) C
  • T_low ≈ 0.693 R2 C

• Monostable (one-shot)

  • Triggered by a negative-going edge on TRIG.
  • Output goes high for a time t.
  • t ≈ 1.1 R C (using the standard configuration: R between Vcc and node, C from node to ground)
  • After the pulse, the capacitor discharges through the DISCH pin.

• Bistable (flip-flop)

  • No RC timing network. TOGGLE output between high/low with proper SET/RESET triggers.
  • Reset pin can force output low (active low).

Tips

• Control voltage pin (5) lets you shift the 2/3 Vcc reference if you need to.
• Use a small series resistor with the control pin if you’re tying it to a voltage other than Vcc/2 to avoid noise sensitivity.

Quick design example

• Design a monostable with t ≈ 5 ms, Vcc = 5 V, using C = 100 nF.
  • R ≈ t / (1.1 C) = 0.005 / (1.1 × 100e-9) ≈ 45.5 kΩ
  • Pick a standard value like R = 47 kΩ and C = 100 nF → t ≈ 1.1 × 47k × 100 nF ≈ 5.2 ms

Would you like:

• A couple of practice problems (e.g., calculate f for a given astable, or design a monostable for a target pulse width)?
• A more course-specific rundown (digital electronics, analog, lab guidance)?
• A step-by-step walkthrough of a particular circuit you’re studying?